Recent work in the field of olefin upgrading has resulted in catalytic processes for converting lower olefins to heavier hydrocarbons. Particular interest is shown in a technique wherein gasoline and/or distillate range hydrocarbons can be synthesized over ZSM-5 type medium pore zeolite catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. In U.S. Pat. No. 4,227,992 Garwood and Lee disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons. In a related manner, U.S. Pat. Nos. 4,150,062 and 4,211,640 (Garwood et al) disclose a process for converting olefins to gasoline components.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10.sup.+ aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions of the prior art do not convert a major fraction of ethylene. A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of nonreactive paraffins and the like being acceptable components for ordinary commercial purposes. These prior art oligomerization methods require relatively high temperature to provide adequate conversion, resulting in undesirable cracking reactions and a broad spectrum of carbon numbers in the products. The product of medium pore olefin catalysis can be characterized as nearly linear, with only moderate branching due to the constrained pores of the catylysts.
While low temperature oligomerization is known, prior catalysts have not shown sufficient activity below about 200.degree. C. to be practical in industrial applications. The advantages of low severity oligomerization with medium pore zeolites have been described by Avidan et al in U.S. Pat. No. 4,746,762 and 4,873,385. It is generally understood that low temperature oligomerization can be selective to produce incremental oligomers which have molecular weights as multiples of the monomers, such as isomeric propene oligomers consisting essentially of C6, C9, C12, etc. These reactions are selective without significant cracking of the desired product; however, the relative inactivity of prior art catalysts has prevented development of low temperature processes.
It is an object of this invention to provide an improved process for selective catalytic oligomerization of an olefinic feedstock which comprises contacting said feedstock under low severity catalytic conversion conditions with a novel acid solid catalyst having ultra-large pores and exceptionally high oligomerization activity at low temperature. The resulting oligomers are highly branched, incremental oligomers, which can be further converted under disproportionation or cracking conditions to yield lower olefins rich in tertiary compounds.
A new catalyst material has been found to have excellent oligomerization properties, especially at low reaction temperature.